K₂(NbO)₂Si₄O₁₂: a new ferroelectric

The atomic coordinates of K₂(NbO)₂Si₄O₁₂ reported in space group P4bm satisfy the structural criteria for ferroelectricity. The estimated phase-transition temperature, T_c = 2230 (250) K, is substantially in excess of T_melting = 1476 (5) K; the hypothetical paraelectric phase in space group P4/mbm is hence experimentally inaccessible. A strong indication that the phase transition does not occur at T < T_melting is provided by the absence of both calorimetric and dielectric permittivity anomalies below T_melting, the linear thermal dependence of unit-cell parameters with expansion coefficients α₁₁ = –0.53 (12) × 10^–6 and α₃₃ = 24.66 (11) × 10^–6 K^–1 between 5 and 843 K, and the generation of second harmonics between 300 and 1353 K. Demonstration of dielectric hysteresis under both direct and alternating current, with spontaneous polarization P_s ≃ 0.2 C m^–2, provides unambiguous verification of the ferroelectric property.